1. Field of the Invention
The present invention relates to a process for developing a dendritic cadmium powder and a powder obtained by the process.
2. Description of the Prior Art
In the general framework of the development of nickel/cadmium accumulators, the tendency is constantly to search for an improvement in performance. Thus the electrode structures are particularly designed so as to be able to contain a charge of active material which is as high as possible (maximum number of ampere-hours) and as available as possible (maximum current).
A particular aspect of research in the field has consisted in developing novel techniques for manufacturing electrodes which employ smaller quantities of these materials.
Thus, especially in the field of portable accumulators, the negative-electrode structure, usually made from sintered nickel, has been replaced by a structure called PBT in which a mixture of cadmium oxide and metallic powder is coated onto a strip. In this known technique, the role of the conductive powder is to distribute the current of electrons uniformly in the volume of the active mass of cadmium hydroxide.
Conventionally, a certain number of metallic powder types are used, either of-cadmium or of nickel. Often spherical or spheroidal, these known powders are added in significant proportions, typically of the order of 20% by weight, in order to achieve the required resistivity for the electrode.
The Japanese Patent Application published under the number 55-76,569 on June 9, 1980 teaches the development of such a powder and its incorporation into the electrode in such proportions.
The present invention is based on the observation according to which, with a powder which deviates from the spherical shape (in other words with a shape factor which is very much greater than 1), and in particular with a dendritic powder, the electrical performance is greatly improved and, in particular, the energy density is increased. This means that, in order to obtain the same performance as with an electrode of the prior art, a dendritic powder is added to the paste of the electrode in a quantity substantially less than a spherical powder, resulting in a weight gain.
Document FR-A-2,194,792 teaches a process for developing a porous electrode from a cadmium powder of acicular or dendritic nature. This powder, obtained by deposition on a electrode and then dry scraping, is then compressed in order to form the electrode and is entirely unsuitable for use as an agent for distributing the current as mentioned hereinabove. More precisely, the operating conditions described in this document are such that the powder does not have the required fineness. Furthermore, the principle of electrolysis described in this patent necessitates working with small quantities of electricity and with extremely frequent scrapings.